feat(hub): big monochrome disc placeholder for covers with no art

Album tiles/detail covers with no embedded art, no sibling image file,
and no MusicBrainz match (or MusicBrainz off) used the same small
music-note icon list rows use elsewhere - fine as a rare fallback, but
most tiles in a real library land here, so it read as broken rather
than as the default look of the grid.

- icon.js: a "disc" icon (two concentric circles, same stroked style
  as the rest of the set) - a plain CD/vinyl glyph.
- music-app.js: AlbumCard and MusicDetailModal's cover now pass
  emptyIcon="disc" instead of "music".
- style.css: sized at 55% of the tile via CSS rather than a fixed px
  value, so it scales with .music-grid's auto-fill columns; color
  stays var(--text-dim), the same neutral tone every other empty state
  already uses - monochrome, not a new accent.

Client-side only. npm run sync-ui re-run. Full suite: 1129 passed, no
regressions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KBi7ALLGfwcjBXt57yNMcy

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